I’ve seen this for a long time and in fact opened a topic to discuss it. The page header will say I have three unread topics, but they disappear when I click. The opposite happens as well, no notification but when I click anyway there are new unread topics in the list.
Two of the forum software developers commented- its not the same problem the user commented on earlier. One of the developer said he knew about it, and he’ll take care of it at some point, but its a difficult problem because its hard to duplicate.